1. Lecture 24

2. Terrestrial planets: atmospheres

3. Atmospheres are created by three processes:
Outgassing
Evaporation/sublimation
Impacts
All planets probably had minimal (primary. H,He) atmospheres at some point after they formed

4. How do you keep an atmosphere?
Gravity attracts gas towards the planet.
Thermal motions of the gas can cause it to escape.
Gravity determines the escape velocity
Temperature determines the thermal velocity.
Large planets hold onto atmospheres easier than small planets
Cooler planets hold onto atmospheres easier than hot ones.

5. Original primary atmospheres (H,He) were swept away from the terrestrial planets early in their life.
Jovian planets still have these primary atmospheres
Present day terrestrial atmospheres are secondary atmospheres, formed primarily by outgassing (mostly carbon dioxide - CO2)
or, in the case of the Earth, a tertiary atmosphere of oxygen and nitrogen (secondary atmosphere modified by life and presence of liquid water)‏

6. Which moons have atmospheres?
Jupiter's Io (tenuous sulfur dioxide)‏
Jupiter's Europa (tenuous oxygen)‏
Saturn's largest moon Titan (dense nitrogen & methane)‏
Neptune's largest moon Triton (tenuous nitrogen & methane).

7. Moon and Mercury
“black sky”
The little atmosphere that exists consists of particles of the solar wind that are temporarily trapped.

8. Mars Very little atmosphere today (CO2)‏
Mars had standing and running water on its surface in the past.
Therefore, it must have had a more substantial atmosphere in the past
Does it have water today? Yes - frozen in polar ice caps and possibly beneath its soil

9. Earth A moderate atmosphere today
Mostly nitrogen (N2), with some oxygen (O2), carbon dioxide (CO2), etc.
Enough to enable liquid water to exist (temperature and pressure adequate)‏
Together the air & water produce erosion

10. Venus
Densest atmosphere of all Terrestrials
Mostly CO2
Temperature at surface hot enough to melt lead
Pressure at the surface ~ 90 times that on Earth
Perpetual cloud cover, sulfuric acid rain

11. How do we account for these differences?
Composition of outgassed gasses
The dominant gasses arising from outgassing were carbon dioxide and water vapor, with minor amounts of nitrogen, sulfer, argon, …
Each terrestrial planet’s outgassed atmosphere was roughly the same.

12. How do we account for these differences?
Composition of outgassed gasses
(Carbon dioxide and water vapor)‏
Temperature appropriate for liquid water?
Distance from the sun, amount of greenhouse gasses in atmosphere.
Oceans absorb carbon dioxide -> carbonate rocks

13. How do we account for these differences?
Composition of outgassed gasses
(Carbon dioxide and water vapor)‏
Temperature appropriate for liquid water?
(Carbon dioxide dissolves in oceans)‏
Interaction of light and atmospheres
Dissociation of water and ammonia molecules by UV light, and warming by the greenhouse effect

14. How do we account for these differences?
Composition of outgassed gasses
(Carbon dioxide and water vapor)‏
Temperature appropriate for liquid water?
(Carbon dioxide dissolves in oceans)‏
Interaction of light and atmospheres
(dissociation and the greenhouse effect)‏
Photosynthetic life - generation of oxygen

16. Interactions of light with atmospheric gasses
X rays
ionize atoms & molecules
dissociate molecules
absorbed by almost all gases
Ultraviolet (UV)‏
dissociate some molecules
absorbed well by O3 & H2O
Visible (V)‏
passes right through gases
some photons are scattered
Infrared (IR)‏
absorbed by greenhouse gases

17. hottest layer, v. rarified Thermosphere
Exosphere
hottest layer, v. rarified
Thermosphere
absorbs X-rays, ionized, ionosphere, reflects some radio, aurora
Mesosphere
weakly absorbs UV
Stratosphere
strongly absorbs UV, ozone (O3), stratified (no convection)‏
Troposphere
absorbs IR (greenhouse); convective; weather

18. The greenhouse effect
Planets heat up by absorbing the Sun’s visible light
Planets cool off by radiating infrared out to space
Greenhouse gasses trap infrared radiation in troposphere (lowest level of atmosphere), heating lower atmosphere.
greenhouse gasses (e.g., H2O, CO2, CH4 - methane) transmit visible light but absorb infrared light

20. Greenhouse effect raises temperature of lower atmosphere
Greenhouse effect is critical to the existence of life on Earth – it raises temperatures to “habitable” level, permits liquid water

21. Terrestrial planets: atmospheres
Question: Why does Venus have so much more atmosphere than Earth?
The answer is found in what Venus’ atmosphere is made of: CO2 (carbon monoxide)‏

22. What happens if there is a lot of CO2 in a planet’s atmosphere?
Due to the large amount of CO2 in its atmosphere, the surface temperature on Venus is over 700 K, instead of the 230 K that it should be at this distance from the Sun.
Does this also explain why Venus has so much atmosphere???
YES!

23. Evolution of Atmospheres: Earth vs. Venus
because water can exist in liquid form
 On Earth there are oceans
Original CO2 has dissolved into oceans,
rocks (carbonates) which keeps levels of CO2 just balanced in atmosphere
 keeps planet WARM but not HOT
if planet were hotter, CO2, H2O would
be boiled out of oceans and baked out of rocks
 more CO2, H2O enter Atmosphere
Evolution of
Atmospheres:
Earth vs. Venus

24. Evolution of Atmospheres: Earth vs. Venus
Liquid water may have existed early in Venus’ history – but most vaporized into
atmosphere: T was hotter on Venus
H2O vapor is a greenhouse gas - trapped
energy making planet hotter; eventually T
so high that water boils
‘runaway’ because more H20 goes into the
Atmosphere as it evaporates; no water left
on planet to dissolve CO2 – out of balance!
eventually stabilized when H20 broken down
by UV sunlight and no further CO2 to bake
out of the Venus surface

25. Earth has about the same amount of CO2 as Venus
Much of the Earth’s CO2 is ‘frozen’ into the rocks
However, if we could raise the temperature of our atmosphere a little bit, it would release a little bit more CO2 into the air
This would trap a little bit more heat, raising the temperature a little bit more…
This would release a little bit more CO2…
…which would trap a little bit more heat…

26. …which would raise the temperature a little bit more…
…which would release a little bit more CO2…
…which would trap a little bit more heat…
You get the idea!

27. This is called the runaway greenhouse effect
It happened on Venus because Venus is closer to the Sun
So - Earth has less atmosphere because most of our CO2 is still frozen in rocks

28. Global warming concern There has been a large Increase in atmospheric CO2 Human activity is responsible for this increase. CO2 is a strong greenhouse gas There is a strong correlation between the increase in carbon dioxide in the atmosphere and the global temperature. Essentially all evidence indicates that human activity is a significant contributor to this trend (50%, 100% ???). Can it be halted? reversed?

29. red) Ozone is broken apart by ultraviolet radiation.

30. What Determines a Planet’s Surface Temperature?
In the absence of the Greenhouse Effect:
the planet's distance from the Sun
the planet’s overall reflectivity
the higher the albedo (reflectivity), the less light absorbed  planet cooler

31. What Determines a Planet’s Surface Temperature?
Greenhouse Effect.
cannot change incoming Sunlight, so it cannot change the total energy returned to space
it increases the energy (heat) in lower atmosphere, keeping the surface warmer
It works like a blanket

32. Greenhouse Effect on the Planets
Greenhouse Effect warms Venus, Earth, & Mars
on Venus: it is very strong
on Earth: it is moderate
on Mars: it is weak
avg. temp. on Venus & Earth would be freezing without it

33. Global warming concern There has been a large Increase in atmospheric CO2 Human activity is responsible for this increase. CO2 is a strong greenhouse gas There is a strong correlation between the increase in carbon dioxide in the atmosphere and the global temperature. Essentially all evidence indicates that human activity is a significant contributor to this trend (50%, 100% ???). Can it be halted? reversed?